Showing papers by "Anvar A. Zakhidov published in 1998"

Carbon Structures with Three-Dimensional Periodicity at Optical Wavelengths

Abstract: Porous carbons that are three-dimensionally periodic on the scale of optical wavelengths were made by a synthesis route resembling the geological formation of natural opal. Porous silica opal crystals were sintered to form an intersphere interface through which the silica was removed after infiltration with carbon or a carbon precursor. The resulting porous carbons had different structures depending on synthesis conditions. Both diamond and glassy carbon inverse opals resulted from volume filling. Graphite inverse opals, comprising 40-angstrom-thick layers of graphite sheets tiled on spherical surfaces, were produced by surface templating. The carbon inverse opals provide examples of both dielectric and metallic optical photonic crystals. They strongly diffract light and may provide a route toward photonic band-gap materials.

Negative Poisson's ratios as a common feature of cubic metals

Abstract: Poisson's ratio is, for specified directions, the ratio of a lateral contraction to the longitudinal extension during the stretching of a material Although a negative Poisson's ratio (that is, a lateral extension in response to stretching) is not forbidden by thermodynamics, this property is generally believed to be rare in crystalline solids1 In contrast to this belief, 69% of the cubic elemental metals have a negative Poisson's ratio when stretched along the [110] direction For these metals, we find that correlations exist between the work function and the extremal values of Poisson's ratio for this stretch direction, which we explain using a simple electron-gas model Moreover, these negative Poisson's ratios permit the existence, in the orthogonal lateral direction, of positive Poisson's ratios up to the stability limit of 2 for cubic crystals Such metals having negative Poisson's ratios may find application as electrodes that amplify the response of piezoelectric sensors

Observation of inhibited spontaneous emission and stimulated emission of rhodamine 6G in polymer replica of synthetic opal

Abstract: We report the observation of inhibited spontaneous emission of organic dye rhodamine 6G infiltrated in a polymer replica of synthetic opal as a photonic crystal. The morphology-dependent resonances, superimposed on the broadband emission of rhodamine 6G due to spherical wavelength-sized microcavity enhancement of dye emission, have also been observed.

Spectral Narrowing of Photoluminescence in Conducting Polymer and Fluorescent Dyes Infiltrated in Photonic Crystal, Synthetic Opal.

Abstract: Spectral narrowing of photoluminescence (PL) has been observed in conducting polymers such as poly(2,5-dialkoxy- p-phenylenevinylene) infiltrated in photonic crystals, synthetic opals, upon light excitation. Such spectral narrowing has also been observed in synthetic opals infiltrated with fluorescent dyes such as Rhodamine 6G and Coumarin. The threshold of the spectral narrowing in conducting polymers and dyes infiltrated in opal is much lower than that in their solutions. Dependence of spectral narrowing characteristics on the lattice periodicity of opals has also been studied. The results are discussed in terms of amplified spontaneous emission and lasing due to optical feedback in opal.

Nanostructured thermoelectrics based on periodic composites from opals and opal replicas. I. Bi-infiltrated opals

Abstract: We have developed a variety of templating processes for the fabrication of three-dimensionally periodic, nanostructured thermoelectrics from opal and inverse opal matrices. These opals and inverse opals are periodic at optical wavelengths and have extremely high interfacial area. It was hoped that scattering processes at the interface between opal and infiltrated thermoelectric material would increase the thermoelectric figure of merit (ZT) by having a greater effect on phonon-mediated (lattice) thermal conductivity than on electronic conductivity. We provide the first demonstration that this approach can increase ZT by evaluating a simple prototype system: bismuth infiltrated into porous SiO/sub 2/ opal. We find a larger fractional decrease in thermal conductivity than for electrical conductivity (relative to bulk polycrystalline Bi). Since the thermopower is little changed, the overall effect we observe is as much as a two-fold increase of ZT compared with that for polycrystalline bulk bismuth. However, the observed ZT is much smaller than for single crystal bismuth.

Photoluminescence, Electroluminescence, Lasing and Novel Characteristics in Photonic Crystal, Synthetic Opal, of Conducting Polymers, Polyacetylene Derivatives

Abstract: Photoluminescence (PL) intensity and spectrum are strongly dependent on substituents of conducting polymers, such as polyacetylene derivatives. Intense PL and electroluminescence (EL) have been observed in di-substituted polyacetylene derivatives. Novel EL characteristics have also been found in EL devices with aligned conducting polymers and also of various electrode configurations. Spectral narrowing and lasing have been observed in conducting polymers with micro-ring structures and also in photonic crystals upon relatively low optical excitation.

Electronic transport properties of Bi/sub x/Te/sub y/-infiltrated synthetic opals

Abstract: We have demonstrated that crystalline Bi/sub x/Te/sub y/ can be grown directly inside the 100-200 nm diameter pores of a synthetic opal host using a metal-organic chemical vapor deposition (MOCVD). A series of Bi/sub x/Te/sub y/-infiltrated opal samples have been characterized by X-ray powder diffraction, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDXS). Thermoelectric power values in the infiltrated opal samples ranged from -70 /spl mu/V/K to +200 /spl mu/V/K at room temperature T=300 K for the Bi- and the Te-rich Bi/sub 2/Te/sub 3/ compositions, respectively. The n-type (Bi-rich) samples were found to exhibit metallic electrical resistivity R (dR/dT>0) from 10 to 300 K, while the p-type (Te-rich) samples demonstrated negative dR/dT.